What the heck is an uncollapsible collapsed layer?
This is now the second day running I've had this error. I do like a nice incomprehensible AE error but this one really doesn't seem to mean anything.

It occurs when I'm pasting footage copied from Premiere into an AE comp. The footage doesn't have anything special on it, it's just UHD in an HD timeline that's been scaled to the frame size. Also, it doesn't seem to result in anything breaking. Any ideas what I'm doing wrong? Here's what the comp looks like afterwards - some of the footage is precomped presumably because it was running backwards in Premiere, but some of it seems to have been precomped for no reason at all.

